Ellipsometry is used to measure the thickness of thin films of, for example, several 100 nm or less. In this method, what is measure is a change in the polarized state when light is reflected by the surface of a specimen such as a thin film, or the ratio .rho. of the reflectivity Rp of a parallel component (P component) to the plane of incidence of electric-field vector to the reflectivity Rs of a perpendicular component (S component), expressed as equation (1). According to an already known specific relationship between polarization reflectivity ratio .rho. and film thickness d, the thickness d of the thin film is calculated as: EQU .rho.=Rp/Rs=tan.psi. exp [j.DELTA.] (1)
The polarization reflectivity ratio .rho. is expressed by two ellipsoparameters, amplitude ratio .psi. and phase difference .DELTA., as shown in equation (1). These two ellipsoparameters are physical quantities obtained by measurement.
To compute these ellipsoparameters .psi. and .DELTA., an 3-channel ellipsometer with no moving parts as shown in FIG. 18 has been developed, as disclosed in Published Unexamined Japanese Patent Application No. 1-28509.
The light of a single wavelength emitted from a light source 1 made up of a laser light source is converted into linearly polarized light by a polarizer 2 and is directed to the surface of a specimen 3, the object to be measured, at an incident angle of .phi.. At the specimen surface 3, the plane of incidence is parallel to the paper on which the figure is drawn. It is assumed that the direction parallel to the paper is direction P and the direction perpendicular to the paper is direction S. The reflected light from the specimen surface 3 is split into three light beams by three nonpolarization beam splitters 4a, 4b, and 4c.
A first light passing through two beam splitters 4a and 4b is directed to a first photodetector 7a via a first analyzer 5a and a first condenser lens 6a. The first photodetector 7a converts the intensity I1 of the first light into an electric signal. Similarly, a second light reflected by beam splitter 4b after passing through beam splitter 4a is directed to a second photodetector 7b via a second analyzer 5b and a second condenser lens 6b. The second photodetector 7b converts the intensity I2 of the second light into an electric signal. A third light passing through beam splitter 4c after being reflected by beam splitter 4a is directed to a third photodetector 7c via a third analyzer 5c and a third condenser lens 6c. The third photodetector 7c converts the intensity I3 of the third light into an electric signal.
The analyzers 5a to 5c pass only optical components oscillating in a preset direction. The polarizing direction of the first analyzer 5a is set to the reference direction (with an angle of 0.degree.); that of the second analyzer 5b is set to a direction inclined +45.degree. with respect to the reference direction; and that of the third analyzer 5c is set to a direction inclined -45.degree. with respect to the reference direction. The reference direction is determined in such a way that a direction (direction P) parallel to the plane of incidence on the specimen surface 3 has an angle of 0.degree., as shown by arrow "a" when viewed from the photodetector 7a.
Therefore, when the light reflected by the specimen surface 3 is polarized elliptically as shown in FIG. 19, the first light intensity I1 at the first photodetector 7a indicates an amplitude of the orthogonal projection of the elliptically polarized light on the abscissa (in the direction with an angle of 0.degree.); the second light intensity I2 at the second photodetector 7b represents an amplitude of the orthogonal projection of the elliptically polarized light on a line inclined +45.degree.; and the third light intensity I3 at the third photodetector 7c shows an amplitude of the orthogonal projection of the elliptically polarized light on a line inclined -45.degree..
The aforementioned ellipsoparameters .psi. and .DELTA. are the amplitude ratio .psi. of component P to component S of the elliptically polarized light reflected from the specimen surface 3 as shown in FIG. 19 and their phase difference .DELTA.. A simple geometrical consideration shows that these ellipsoparameters .psi. and .DELTA. can be computed by equations (2) and (3): EQU cos(.DELTA.-.phi..sub.i) =(I2-I3)/(2I1){I1/(I2+I3-I1)}.sup.1/2( 2) EQU tan.psi.=.sigma..sup.2 tanp{I1/(I2+I3-I1)}.sup.1/2 ( 3)
where the phase difference .phi..sub.i and amplitude ratio P are the ellipsoparameters of the incident light: for example, for a linearly polarized light of +45.degree., .phi..sub.i =0.degree., tanp=1; for a linearly polarized light of -45.degree., .phi..sub.i =0.degree., tanp=-1. .sigma. is a unique value determined by the reflectivity in each direction of these beam splitters 4a to 4c. The .sigma. is obtained in advance by directing known elliptically polarized test light to the respective beam splitters 4a to 4c.
Once ellipsoparameters .psi. and .DELTA. are obtained, the film thickness d can be computed using suitable equations.
With such an ellipsometer without moving parts, it is possible to measure at a high speed of 1000 points per second or more.
The number of parts in the optical system, however, is very large as shown in FIG. 18. To split the reflected light from the specimen surface 3 into the first, second, and third light, three beam splitters 4a to 4c are required. In addition, analyzers 5a to 5c have to be placed after the beam splitters 4a to 4c, respectively. Those optical members must be set precisely in terms of mutual angles including their solid angle. To achieve this, each of beam splitters 4a to 4c needs at least a thickness of 1 cm or more, and a height and length of nearly 5 cm. Thus, those beam splitters 4a to 4c alone occupy an area of approximately 15 cm.sup.2.
The rotational angle of each of analyzers 5a to 5c have to be measured accurately before the thickness d of oxide films or the like is actually measured using the ellipsometer. This makes the rotational driving mechanisms of analyzers 5a, 5b, and 5c complicated. For instance, the analyzer had to be rotated 45.degree. during the initial setting. Because of this, a single analyzer had to be as large as approximately 5 cm.sup.2. To keep the analyzers away from each other, an installation place with a lateral direction of approximately 15 cm was needed.
Since the specimen surface 3 is far away from the respective photodetectors 7a to 7c, the reflected light from the specimen surface 3 spreads until it reaches the photodetectors 7a to 7c. For this reason, it is necessary to use condenser lenses 6a to 6c, theoretically unnecessary, to gather rays of light on the respective photodetectors 7a to 7c. As a result, the number of optical parts in the whole ellipsometer increase even more.
Since light attenuates when passing through each of beam splitters 4a to 4c, the light intensities I1 to I3 received by the photodetectors 7a to 7c become smaller, resulting in poorer S/N ratio. To avoid this problem, it is necessary to use, for example, a high-power laser device as the light source 1.
As noted above, in the conventional ellipsometer of FIG. 18, to assure the accuracy of the optical system, it is necessary to make the entire system more rigid and larger. Even when the smallest optical parts are used, a system composed of a light source section made up of the light source 1 and the polarizer 2 and a sensing section made up of the beam splitters 4a to 4c, photodetectors 7a to 7c, and related parts, requires an area of 50 cm.sup.2 and a height of approximately 50 cm. It weighs 30 to 50 kg.
Such a large, heavy ellipsometer requires installation in a room such as a laboratory, and a specimen to be measured must be carried to the installation place.
In various manufacturing lines in the today's factory, there is an increasing need of monitoring the thickness of coating through on-line measurement of various types of paint or oil coated over the surface of a variety of belt-like products conveyed over the manufacturing lines.
In practice, however, it is impossible to install in such a manufacturing line an ellipsometer containing many optical members as well as a driving mechanism, as noted earlier.